This example code:

let 𝓢𝓝𝓐𝓐𝓐𝓐𝓚𝓔𝓢 = 1;

Produced a useless conversion suggestion:

warning: variable `𝓢𝓝𝓐𝓐𝓐𝓐𝓚𝓔𝓢` should have a snake case name
 --> src/main.rs:4:9
  |
4 |     let 𝓢𝓝𝓐𝓐𝓐𝓐𝓚𝓔𝓢 = 1;
  |         ^^^^^^^^^ help: convert the identifier to snake case: `𝓢𝓝𝓐𝓐𝓐𝓐𝓚𝓔𝓢`
  |
  = note: `#[warn(non_snake_case)]` on by default

The problem here is that while these characters are in the Uppercase Letter (Lu) general category, they do not have a proper lowercase mapping: the lowercase of 𝓐 is still 𝓐, not 𝓪 (thus Rust cannot suggest 𝓼𝓷𝓪𝓪𝓪𝓪𝓴𝓮𝓼).

This is the same for the other direction:

warning: type `𝕟𝕠𝕥_𝕒_𝕔𝕒𝕞𝕖𝕝` should have an upper camel case name
 --> src/lib.rs:3:8
  |
3 | struct 𝕟𝕠𝕥_𝕒_𝕔𝕒𝕞𝕖𝕝;
  |        ^^^^^^^^^^^ help: convert the identifier to upper camel case: `𝕟𝕠𝕥𝕒𝕔𝕒𝕞𝕖𝕝`
  |
  = note: `#[warn(non_camel_case_types)]` on by default

It shouldn't produce the suggestion at all if the conversion result is not changed, but I'm not sure about cases involving adding or removing underscores.

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rustc on playground, 1.48.0-nightly (2020-09-26 623fb90).